The present invention relates to generally a magnetic television signal recording and/or reproducing system of the type in which rotating heads record the video signal in a series of oblique tracks on magnetic tape and more particularly a system for recording and/or reproducing the audio signal associated with the video signal.
Rotating head type magnetic video tape recorders are widely used for simultaneously recording and reproducing the video and audio signals. In general, the video signal is recorded and reproduced with rotating magnetic heads while the audio signal is recorded and reproduced with a stationary magnetic head.
Because of improvements in the quality of magnetic tapes and magnetic heads, advancement in signal processing techniques and control systems and the increase in accuracy in various components and mechanisms, the recording density of video signals has been remarkably increased. For instance, VHS system video tape recorders with a recording time of 4 hours have a recording density about 92 times as high as that of the video tape recorders used in broadcasting stations of the type having 4 heads and using magnetic tape 2 inches in width and about 11 times as high as that of EIAJ type I video tape recorders. VHS video tape recorders use magnetic tape of the 1/2 inch width which is transported at 1.65 cm/sec. Qualities and characteristics of tape and heads will be further improved, so that the recording density will be further increased.
For instance, assume that the present recording density is doubled. Then when 1/2 inch-tape is used, the transport speed will become about 0.8 cm/sec. When the transport speed is reduced to such an extremely low speed, the conventional audio signal recording systems with a stationary head cannot attain high-quality recording and reproduction because of the following reasons.
(a) As the transport speed is reduced, the recorded wavelengths of the audio signal becomes short, so that the recording and reproduction of the high-frequency audio signal becomes extremely difficult. Furthermore, a sufficient audio signal bandwidth (higher than 10 kHz) cannot be obtained. (When the tape transport speed is 1 cm/sec, the highest or upper limit is 5 kHz at the present state of the art.)
(b) As the tape transport speed is decreased, the output from a reproduce head drops; S/N is degraded.
(c) As the tape transport speed is reduced, the signal recording level is lowered and the dynamic range becomes narrow, so that distortions result.
(d) Accuracies of components and mechanisms are limited, so that wow and flutter are increased.
Therefore, even though the recording density of the video signal will be much increased, an increase in recording density of the audio signal will not result, because of the reasons described above, so that an increase in recording density of the television signal will be extremely difficult.
It may be proposed to use magnetic tapes 1/4 or 1/8 inch in width in order to increase the recording density of the video signal without reducing the tape transport speed. However, magnetic tape of 1/4 inch in width must be twice as long as magnetic tape of 1/2 inch in width in order to record the same amount of information at the same recording density. As a result, even though tape cassettes may be reduced in thickness to some extent (the thickness is reduced not to a half but two 2/3 because of the thickness of the case and the thickness of the reel hub remain unchanged and the space for housing the reels and the case also remains unchanged), the surface area would be increased. As compared with a 1/2 inch-tape cassette capable of recording for 2 hours, a 1/4 inch-tape cassette will become very large in size and unbalanced. If a 1/8 inch-tape is used, a relatively high tape transport speed could be secured for satisfactory audio signal recording. However, the surface area of a tape cassette would become by far larger and unbalanced.
In addition to the above-described problems, there arise other problems in the case of recording and reproducing the video signal. That is, when tape width is reduced, skew distortions (discontinuity in time of the signal when heads are switched) would result due to expansion and compression of the tape. Furthermore, the angle of inclination of the oblique tracks relative to the tape center line would be decreased, so that waving results. As a result, the tape which is recorded by one video tape recorder cannot be reproduced by another. In addition, satisfactory air films cannot be formed between a rotating cylinder and a tape, so that the tape transport would be adversely affected and consequently jitter results. Thus, from the standpoint of recording and reproduction of the video signal, the use of tape with a larger width is advantageous.
As described above, with the conventional audio signal recording and reproducing systems with a stationary head, the problem of further increasing the recording density of the video tape recorders while maintaining tape cassettes in balanced shape cannot be solved.
In order to overcome these problems, there has been proposed a system which is used in conjunction with the video disk systems and in which the audio signal is frequency modulated at a center-frequency lower than the lower limit of the frequency-modulated video signal band and multiplexed with the FM video signal for recording. However, when such frequency multiplexing is employed, the additional recording of the audio signal (to be referred to as "after-recording" in this specification) cannot be attained. Such a multiplexing system may be advantageous for the video disk systems because their sole function is to reproduce the prerecorded video and audio signals, but cannot be used in the video tape recorder which, as its name implies, not only records the video and audio signals but also reproduces them.
In order to attain high-density recording and to permit the "after-recording" of the audio signal, there has been proposed a system in which the audio signal is time compressed and then recorded by means of rotating heads which are used for recording or reproducing the video signal. However, the audio signal compression and expansion circuits need an extremely large number of components so that there arise the problems that the video tape recorders become large in size and heavy in weight and the power consumption is increased. These problems may be negligible in the case of the home-use video tape recorders, but they are very serious in the case of the portable video tape recorders.